The present invention is concerned with a controlled release tablet dosage formulation that is based on the use of a tablet core which has a coating which contains a major amount of a water insoluble polymer and a minor amount of a water soluble compound.
In order to provide a controlled release product, a water insoluble material such as a resin or a wax has been used to coat discrete drug containing units in order to resist the action of the fluids in the gastrointestinal tract. These coatings have resulted in the production of satisfactory pellets or granules that would usually be placed in a gelatin capsule because if a tablet was made from the resin or wax coated pellets, the coating would either rupture or become so deformed that the uniformity of any wax coating would be adversely affected.
U.S. Pat. No. 3,977,404 discloses that in the prior art, attempts were made to make sustained release devices by placing a water soluble drug in a polymer film and allowing the drug to be leached out. Another attempt was based on the use of a drug that was coated with a film of water insoluble plastic containing a modifying agent which was soluble at a certain pH. This device would form a porous film under the proper pH conditions which would allow gastrointestinal fluid to dissolve the drug and leach it outwards through the pores in the film. The pH dependent nature of the film made it difficult to control the rate and site of release because of variations in the pH of the gastrointestinal tract.
Osmotic tablets are described in U.S. Pat. No. 3,977,404 which are based on the use of a microporous core which is coated with a material that does not allow for the passage of a drug. These tablets are provided with a pre formed osmotic delivery orifice which allows gastrointestinal fluid to flow into the osmotic core and cause the osmotic core to deliver the contents of the core over an extended period of time. Other variations on this concept are described in U.S. Pat. No. 4,687,660 which describes the use of a polymer coating having a water soluble pore forming material therein which surrounds a core which contains a water soluble enhancing agent.